The present invention relates to a process for the purification of organometallic compounds or heteroatomic organic compounds with hydrogenated getter alloys.
Organometallic compounds are characterized by the presence of a bond between one metal atom (also arsenic, selenium or tellurium being included among metals) and one carbon atom being part of an organic radical such as, for example, aliphatic or aromatic, saturated or unsaturated hydrocarbon radicals; by extension, with the definition of organometallic compounds also the compounds including metal atoms bound to organic radicals by means of an atom other than carbon, such as for instance the alcoholic radicals (xe2x80x94OR) or of esters (xe2x80x94Oxe2x80x94COxe2x80x94R) are meant.
The heteroatomic organic compounds (also simply defined heteroatomic in the following) are those organic compounds comprising, further to carbon and hydrogen, also atoms such as oxygen, nitrogen, halides, sulfur, phosphorus, silicon and boron.
Many of these compounds have been used for a long time in traditional chemical applications. Reagents having very high purity are not generally requested in this field, and their purification is carried out by techniques such as distillation (optionally at reduced pressure, in order to reduce the boiling temperature and therefore the risks of thermal decomposition of the compounds) or re-crystallization from solvents.
However, these compounds have been recently used in high technology applications, particularly in the semiconductor industry. In these applications, the organometallic compounds and the heteroatomic compounds are used as reagents in the processes of chemical deposition from the gaseous state (known in the field with the definition xe2x80x9cChemical Vapor Depositionxe2x80x9d or with the acronym CVD). In these techniques, a gas flow of one or more organometallic or heteroatomic compounds (or a flow of a carrier gas containing a known concentration thereof) is conveyed into a process chamber; then, inside the chamber the compounds are decomposed or reacted, so that materials containing metal atoms or heteroatoms are formed in situ (generally in the form of thin layers). The organometallic or heteroatomic compounds can be already in the gaseous form, but they can also be in the liquid form. In this second case, the gaseous flow of the compound is obtained either by evaporating the compound, in which case the flow is composed only of the compound of interest, or by bubbling a gas in the container for the liquid, in which case the flow contains vapors of the compound in the carrier gas.
The main organometallic gases used in these applications are hafnium tetra-t-butoxide, trimethylaluminum, triethylaluminum, tri-t-butylaluminum, di-i-butylaluminum hydride, dimethylaluminum chloride, diethylaluminum ethoxide, dimethylaluminum hydride, trimethylantimony, triethylantimony, tri-i-propylantimony, tris-dimethylamino-antimony, phenylarsine, trimethylarsenic, tris-dimethylamino-arsenic, t-butylarsine, barium bis-tetrameitylyheptanedionate, bismuth tris-tetramethylheptanedionate, dimethylcadmium, diethylcadmium, iron pentacarbonyl, iron bis-cyclopentadienyl, iron tris-acetylacetonate, iron tris-tetramethylheptanedionate, trimethylgallium, triethylgallium, tri-i-propylgallium, tri-i-butylgallium, trimethylindium, triethylindium, ethyldimethylindium, yttrium tris-tetramethylheptanedionate, lanthanum tris-tetramethylheptanedionate, magnesium bis-methylcyclopentadienyl, magnesium bis-cyclopentadienyl, magnesium bis-tetramethylheptanedionate, dimethylmercury, dimethylgold acetylacetonate, lead bis-tetramethylheptanedionate, bis-hexafluorocopper acetylacetonate, copper bis-tetramethylheptanedionate, dimethylselenium, diethylselenium, scandium tris-tetramethylheptanedionate, tetramethyltin, tetraethyltin, strontium bis-tetramethylheptanedionate, tantalum tetraethoxy-tetramethylheptanedionate, tantalum tetramethoxytetramethylheptanedionate, tantalum tetra-i-propoxytetramethylheptanedionate, tantalum tri-diethylamido-t-butylimide, diethyltellurium, di-i-propyltellurium, dimethyltellurium, titanium bis-i-propoxy-bis-tetramethylheptanedionate, titanium tetradimethylamide, titanium tetradiethylamide, dimethylzinc, diethylzinc, zinc bis-tetramethylheptanedioniate, zirconium tetra-tetramethylheptanedionate, zirconium tri-i-propoxy-tetramethylheptanedionate and zinc bis-acetylacetonate.
The principal heteroatomic compounds used in these applications are trimethylborane, asymmetric dimethylhydrazine (that is, wherein both methyl groups are bound to the same nitrogen atom), t-butylamine, phenylhydrazine, trimethylphosphorus, t-butylfosfine and t-butylmercaptane.
Some typical examples of application of these methods are the production of the semiconductors of type III-V, such as GaAs or InP, or of type II-VI such as ZnSe; the use for p doping (for instance with boron) or n doping (for instance with phosphorus) of traditional silicon-based semiconductors; the production of materials having a high dielectric constant (for example compounds such as PbZrxTi1xe2x88x92xO3) used in ferroelectric memories; or the production of materials having a low dielectric constant (such as SiO2) for isolating electric contacts in semiconductor devices.
For these applications reagents having an extremely high purity are required, with levels of the order of 10xe2x88x921-10xe2x88x922 ppm, whereas the traditional chemical techniques do not allow to obtain levels of impurities lower than about ten ppm. Further, even in the case that organometallic or heteroatomic compounds having high purity are produced, the storage is source of contamination due to gas release from the container walls, which anyway makes necessary to use a purifier immediately before the application (so-called xe2x80x9cpoint-of-use xe2x80x9d purifiers).
U.S. Pat. No. 5,470,555 describes the removal from organometallic compounds of oxygen gas which is present as an impurity, by using of a catalyst formed of copper or nickel metals, or the relevant oxides activated by reduction with hydrogen, deposited on a support such as alumina, silica or silicates. According to the patent, by this method the removal of oxygen gas from a flow of the organometallic compound can be obtained, to values of 10xe2x88x922 ppm.
However, oxygen is not the only impurity that has to be removed from the organometallic or heteroatomic compounds. Other harmful impurities in the CVD processes are for example water and, particularly, the species deriving from the alteration of the same organometallic or heteroatomic compound, following to undesired reactions generally with water or oxygen. For instance, in the case of a generic organometallic compound MRn, wherein M represents the metal, R an organic radical and n the valence of the metal M, contamination from MRnxe2x88x92x (xe2x80x94OR)x species can occur, wherein x is an integer varying between 1 and n. These oxygenated species are harmful in the CVD processes because they introduce oxygen atoms into the material being formed, thus sensibly altering the electric properties thereof.
Object of the present invention is providing a process for the purification of organometallic compounds or heteroatomic organic compounds from oxygen, water and from the compounds derived from the reaction of water and oxygen with organometallic or heteroatomic compounds whose purification is sought.
This object is obtained according to the present invention with a process wherein the organometallic or heteroatomic compound to be purified is contacted with a hydrogenated getter alloy. The purification can be carried out on the organometallic or heteroatomic compound both in the liquid and in the vapor state.
It is also possible to use, in addition to the getter alloy, other impurity sorbing materials, such as palladium on porous supports or a mixture of iron and manganese supported on zeolites.
The use of getter alloys for the purification of noble gases, nitrogen or hydrogen to be used in the microelectronic industry is known. Further, it is known from patent EP-B-470936 the use of hydrogenated getter alloys for the purification of simple hydrides, such as SiH4, PH3 and AsH3.
However, it has been found that a hydrogenated getter alloy is also capable of removing water and oxygen from an organometallic or heteroatomic compound (liquid or as a vapor, pure or in a carrier gas), and of converting the species containing oxygen of the type MRnxe2x88x92x(xe2x80x94OR)x to the original compound or to compounds of the type MRnxe2x88x92xHx which are not harmful to the CVD processes because they do not contain oxygen.